1. Field of the Invention
The present invention relates generally to optical modulators, and more specifically to the linearization of external electro-optical modulators.
2. Description of the Prior Art
Telecommunication systems using optical fiber components are becoming increasingly popular. Basic components of an optical fiber system may include a laser diode, a modulator and a photodetector diode. Modulators are either direct (modulating the optical wave as it is being generated at the source) or external (modulating the optical wave after it has been generated). The design of laser diodes and photodetector diodes is known. However, a major problem with an optical communication system is its dynamic range, which is largely determined by the distortion from the modulator.
An electro-optical modulator modulates the optical wave with an electromagnetic or RF signal. The RF signal interacts with the optical wave over a pre-determined distance. The construction of optical modulators slows the RF wave relative to the optical wave so that it takes the RF wave a longer period of time to travel the interaction distance. Therefore, the electric field, which modulates the optical wave, varies along the interaction distance. Since the RF wave does not act on the same portion of the optical wave throughout the interaction distance, the optical wave is distorted. The longer the interaction distance, the greater the distortion.
There have been many attempts to decrease modulator distortion. One solution is to decrease the interaction distance. However, this attempted solution requires an extremely large electric field to obtain the same phase shift in the optical wave.
Another solution to reduce modulator distortion is to use a compensation means. After an initial interaction between the RF wave and the optical wave, the two waves are separated, a method of compensating for the phase difference is employed and the waves are brought together to interact again. This process would be repeated many times. An idling method, in which the RF voltages in the RF carrying electrodes are switched in a periodic or in an aperiodic fashion, is one method of compensating for the phase differences. For example, in a modulator of the Mach-Zehnder configuration, a 180.degree. phase shift is needed between the two signal branches. This method is inefficient since a very high voltage is needed to come to extinction. Further, these compensation attempts, although capable of working at high frequencies, are either not broadband or suffer from irregular frequency response. Most methods to compensate for the difference in speed between the RF wave and the optical wave have been unwieldy, impractical or expensive to implement.